The present invention relates to conformable, flush-mounted antenna which produces end-fire radiation along the surface, and which is steerable in one or two dimensions.
The prior art includes a pending application of D. Sievenpiper, E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d U.S. provisional patent application, serial No. 60/079,953, filed on Mar. 30, 1998 which relates to a high-impedance or Hi-Z surface.
It is also known in the prior art to place a conformable end-fire antenna or array on a Hi-Z surface. It has been shown that the Hi-Z material can allow flush-mounted antennas to radiate in end-fire mode, with the radiation exiting the surface at a small angle with respect to the horizon.
The Hi-Z surface, which is the subject matter of U.S. patent application serial No. 60/079,953 and which is depicted in FIG. 1a, includes an array of resonant metal elements 12 arranged above a flat metal ground plane 14. The size of each element is much less than the operating wavelength. The overall thickness of the structure is also much less than the operating wavelength. The presence of the resonant elements has the effect of changing the boundary condition at the surface, so that it appears as an artificial magnetic conductor, rather than an electric conductor. It has this property over a bandwidth ranging from a few percent to nearly an octave, depending on the thickness of the structure with respect to the operating wavelength. It is somewhat similar to a corrugated metal surface 22 (see FIG. 1b), which has been known to use a resonant structure to transform a short circuit into an open circuit. Quarter wavelength slots 24 of a corrugated surface 22 are replaced with lumped circuit elements in the Hi-Z surface, resulting in a much thinner structure, as is shown in FIG. 1a. The Hi-Z surface can be made in various forms, including a multi-layer structure with overlapping capacitor plates. Preferably the Hi-Z structure is formed on a printed circuit board (not shown in FIG. 1) with the elements 12 formed on one major surface thereof and the ground plane 14 formed on the other major surface thereof Capacitive loading allows the resonance frequency to be lowered for a given thickness. Operating frequencies ranging from hundreds of megahertz to tens of gigahertz have been demonstrated using a variety of geometries of Hi-Z surfaces.
It has been shown that antennas can be placed directly adjacent the Hi-Z surface and will not be shorted out due to the unusual surface impedance. This is based on the fact that the Hi-Z surface allows a non-zero tangential radio frequency electric field, a condition which is not permitted on an ordinary flat conductor. In one example, a flared notch antenna was placed on a Hi-Z surface, such that the metal shapes making up the antenna are oriented parallel to the surface, as shown in FIG. 2. The antenna exhibits end-fire radiation, in which the radio waves are emitted with the electric field being tangential to the surface, in the form of a leaky TE surface wave.
The radiation pattern for the flared notch antenna on the Hi-Z surface is shown in FIG. 3, along with the pattern for a similar antenna on a flat metal surface. On the Hi-Z surface, the radiation is emitted at 30 degrees to the horizontal, compared to 60 degrees on the metal surface. This suggests that by changing the surface impedance, one can steer a beam in elevation over a range of at least 30 degrees. Tunable impedance surfaces can be made using a variety of mechanical and/or electrostatic techniques, as described in the two patent applications identified above.
It has been determined that the angle at which radiation leaves or is received by an antenna placed about 2.5 cm above a Hi-Z surface depends upon the impedance of the surface. As described in the two U.S. patent applications identified in the immediately preceding paragraph, this surface impedance can be tuned in real time using a variety of techniques. When used with an end-fire array antenna, the antenna can be steered in two dimensions. The antenna is conformable and aerodynamic and can be readily incorporated into the outer skin of an aircraft or other vehicle. Such an antenna can be flush mounted on the exterior walls or rooftops of buildings to provide scanning over a wide angle. Additionally, conformable flush-mounted antennas are useful for automobiles for the reception of cellular signals, personal communication service (PCs) voice and digital data, collision avoidance information, or other data.
In general terms the invention provides a steerable antenna for receiving and/or transmitting a radio frequency wave, the antenna comprising a tunable high impedance surface; and at least one end-fire antenna disposed on said surface.
In another aspect, the invention provides a method of steering a radio frequency wave received by and/or transmitted from an antenna, the method comprising: providing a tunable high impedance surface; disposing at least one end-fire antenna on said surface; and varying the impedance of the tunable high impedance surface.